Wideband quasi-nondiffracting radial GRIN lenses

We propose a quasi-closed-form refractive-index profile for flat radial gradient index (GRIN) lenses that allows the generation of direction-steerable quasi-nondiffracting beams in a wide frequency range. The GRIN profile is derived so that the optical path lengths of rays reaching the lens aperture are linearly correlated, leading to the generation of quasi-nondiffracting beams within a rhombic region near the lens. Detailed phase error analysis demonstrates that the proposed formulation is valid for a wide range of GRIN lens parameters, including the maximum refractive index, the specified maximum edge phase (which determines the nondiffracting range of the lens), the focal distance, and the thickness of the lens. The validation of the proposed GRIN profile is tested by both full-wave simulations and measurements of a 3-D printed prototype. Experimental results reveal that the designed GRIN lens generates a 2-D scanning quasi-nondiffracting beam with a scanning range of ±25.6° in elevation angle and full azimuth coverage from 26 to 40 GHz, spanning the entire Ka-band.